The present invention relates to the field of fluid control valves, in general, and more particularly to electrically controlled, shape memory alloy element actuated fluid control valves.
In general, manufacturing processes, like those involved in the semiconductor industry, for example, use fluid control valves in the liquid or gas delivery systems thereof. Typically, these valves are either pneumatically controlled or hydraulically controlled. A present obstacle in the use of the fluid control valves is the surge of flow associated with the rapid opening of the valve. The resulting turbulence and rapid pressure rise in the exiting fluid is undesirable for other system components. For example, in the semiconductor industry such turbulence and rapid pressure rise can cause particle "stir up" that can lead to contamination deposits on the wafers, which causes high rejection rates (i.e., low yields). As a result, several different methods have been used to better control the rate of opening of the valve. Among these are the use of a variable orifice which allows the valve piston to be driven at a slower rate, the use of a solenoid to control the flow of the fluid to the air operator, and the use of a metering valve to limit the fluid flow in the delivery system.
Recently, electrically driven fluid control valves utilizing a shaped memory alloy (SMA) drive element have been proposed for use in the fluid delivery systems of manufacturing processes. Shape memory alloys are materials that arc capable of large and repeatable phase-transformation induced strains. One such valve integrates a single shape memory alloy wire into its valve housing within the biasing spring portion thereof. As proposed, the single SMA wire is essentially a rod having a diameter of approximately one-quarter of an inch. A special power supply with low voltage and high current requirements would be required to heat such a large diameter/mass of wire or rod. In addition, once heated the large mass of material would cool very slowly resulting in an undesirable slow closing of the valve. Another type of SMA driven fluid control valve provides for an SMA wire wrapped around the body of the valve but still integral to the valve. Both of these types provide for mechanically active SMA wire terminations which may lead to mechanical and/or electrical malfunctions. None of these proposed SMA actuated fluid control valves appear to offer commercially viable solutions to the aforementioned concerns with pneumatically or hydraulically driven fluid control valves presently used.
The present invention includes aspects which overcome the drawbacks of the prior proposed SMA actuator fluid control valves and offers further aspects not as yet considered in the prior art.